1. Field of the Invention
The present invention relates to a rubber composition. In particular, the present invention relates to a rubber composition suitable for automobile hoses, for example, gasoline fuel hoses, fuel-cell vehicle hoses (methanol fuel hoses and hydrogen fuel hoses), engine cooling system hoses (radiator hoses, heater hoses, etc.), and refrigerator coolant transport hoses.
2. Description of the Art
Hitherto, in vehicles such as automobiles, engine cooling system hoses, such as radiator hoses used for connection of engines and radiators, and heater hoses used for connection of engines and heater cores, for example, such as shown in the FIGURE have been used. This hose is composed of an inner rubber layer 1, a reinforcement layer 2, and an outer rubber layer 3. The aforementioned hose is produced by the steps of braiding reinforcement fibers on an outer peripheral surface of the inner rubber layer 1 so as to form the reinforcement layer 2, coating the surface thereof with an adhesive, forming the outer rubber layer 3 thereon, and vulcanizing. The interface between the inner rubber layer 1 and the reinforcement layer 2 may be coated with the adhesive.
However, regarding the aforementioned conventional hose, since the reinforcement layer 2 and the inner rubber layer 1, or the reinforcement layer 2 and the outer rubber layer 3, are adhered using an adhesive, there may be a problem in that adhesive force between the reinforcement layer 2 and the inner rubber layer 1, or the reinforcement layer 2 and the outer rubber layer 3, is insufficient due to unevenness in the coating of the adhesive, etc., and therefore, sealing performance is insufficient. Furthermore, since the step of applying a coating of the adhesive is necessary, there are problems in that the manufacturing process is complicated which thereby increases costs. Additionally, the pot life of the adhesive is a concern, and the concentration of the adhesive must be controlled, and as a consequence, stability in productivity may be degraded. In addition, since an organic solvent, for example, toluene, is used as a dilution solvent for the adhesive, there are problems of environmental pollution, etc.
As the materials for forming the aforementioned inner rubber layer 1 or outer rubber layer 3, sulfur-vulcanized rubber compositions including an ethylene-propylene-diene terpolymer (EPDM) as a base material typically have been used. In this sulfur-vulcanized rubber composition, it has been required to blend zinc oxide (zinc white) as a vulcanization accelerating aid. However, the aforementioned zinc oxide is present in the form of zinc salts in the vulcanized material after the vulcanization reaction of the rubber composition. Then, these zinc salts gradually migrate to the surface of the vulcanized material, so that they are eluted into a cooling liquid. Once in the cooling liquid, the zinc salts may react with phosphoric acid components contained in the cooling liquid so as to produce insoluble compounds. As a consequence, problems occur since the insoluble compounds deposit on the inner wall surface of the hose and may cause the hose to clog, and the compounds also deposit at the seal portion between the hose and a connection pipe so as to cause leakage of the cooling liquid.
The present invention was made in consideration of the aforementioned circumstances. Accordingly, it is an object of the present invention to provide a rubber composition which can achieve superior adhesion force between it and a material to be adhered without a coating of an adhesive.
In order to achieve the aforementioned object, a rubber composition according to the present invention includes the following components (A) to (D):
(A) a rubber composed of at least one of an ethylene-propylene-diene terpolymer and an ethylene-propylene copolymer,
(B) a peroxide vulcanizing agent,
(C) a resorcinol-based compound, and
(D) a melamine resin.
The inventors of the present invention earnestly researched in order to omit the step of applying a coating of the adhesive, and conceived of the concept of kneading an adhesive component into the rubber composition. Therefore, further research was performed regarding the combinations of rubbers and adhesive components which have superior adhesion properties. As a result, EPDM and an ethylene-propylene copolymer (hereafter abbreviated as EPM), which were relatively low cost among rubber materials, were considered, and it was discovered that when specific adhesive components, i.e., a resorcinol-based compound and a melamine resin, were kneaded into the aforementioned copolymer, and these components were vulcanized using a peroxide vulcanizing agent, superior adhesive force between the copolymer and a material to be adhered could be achieved, and thus the present invention was realized. The reason the rubber composition according to the present invention has superior adhesive force between it and a material to be adhered thereto is believed to be as described below. That is, it is believed that the aforementioned resorcinol-based compound primarily functions as an adhesive, and the aforementioned melamine resin primarily functions as an adhesive adjuvant, and since the resorcinol-based compound is provided with CH2O from the melamine resin, this makes a covalent bond with the material to be adhered so as to improve the adhesive force. For example, it is believed that the resorcinol-based compound (C) represented by the following general formula (I) is provided with CH2O from the melamine resin so as to have a structure represented by the following general formula (II), and this structure forms a covalent bond with a polyamide bond (xe2x80x94CONHxe2x80x94) of the material to be adhered (for example, a polyamide resin), as is shown by the following reaction formula (III), so as to adhere firmly thereto. Furthermore, a portion of hydroxyl groups of the resorcinol-based compound forms hydrogen bonds with the material to be adhered (for example, a polyamide resin), and it is believed that these hydrogen bonds also contribute to improve the adhesive effect. 
When the weight ratio of the resorcinol-based compound (C) to the melamine resin (D) is within a specified range, the adhesive force between the rubber composition and the material to be adhered is improved further.
When the weight ratio of the resorcinol-based compound (C) relative to the specified rubber (A) is within a specified range, the adhesive force between the rubber composition and the material to be adhered also is improved.